The beginning of fatigue cracks is a very localised surface phenomenon which frequently involves only the first layer of grains adjoining the surface.
Various works, namely that of FORSYTH shows that the beginning of cracks occurs from a polished surface in two stages: an initial stage during which there is plastic deformation of the surface and a propagation stage referred to as I which corresponds to the development of a microcrack by shearing in the first grain or grains underlying the surface.
The beginning of cracks is generally studied by metallography in spite of the excessively pinpoint nature of the observations made by this method, which is not suitable for observing the damage as a whole.
Attempts have been made for a long time to use acoustic, electromagnetic or holographic methods, without one of these methods providing absolutely new results as regards the starting mechanism.
In the field of acoustic methods, the properties of surface localisation of Rayleigh waves seemed to mark them out more particularly for this type of application. However, the tests made by TRUELL and CHICK using attenuation of Rayleigh waves remained without consequence, possibly due to the lack of sensitivity of their method (ultrasonic methods for the study of stress cycling effects in metals--NADD TR 60-920, April 1961). The same was true for the work of BROSSENS, HAKIMI and KHABBAZ, who worked at frequencies which were too low (2 MH.sub.z), ("Detection of fatigue damage with Rayleigh waves", August 1960-Technical Report 60-307, Applied Mechanics Laboratory, M.I.T. Cambridge, Massachusetts. Aeronautica Research Laboratory, Contract No. AF 33 (616) 6469 Project No. 7063, Task No. 70532).
The work carried out by the applicants has made it possible to remove most of the difficulties encountered in the use of surface waves and to propose a method for the detection of the phenomena of corrosion or fatigue cracking of metals by studying variations in the propagation or attenuation speed of Rayleigh waves.
In particular, this work has underlined the fundamental advantage of the acoustic attenuation parameter for measuring fatigue. However, if the method recommended in this way, namely a method of forming a loop from undamped surface waves of high frequency, of the order of 20 MHz, has proved very sensitive, making it possible to measure variations in the speed of the phase of Rayleigh waves as low as 10.sup.-6 and variations of attenuation of some 0.001 dB, it had many drawbacks.
First of all, it was discontinuous, the measurement of the acoustic attenuation taking place in the inoperative state, on the test-piece in the position of maximum stress.
This method has also proved difficult owing to the necessity of careful stabilization of temperature to within 1.degree./100.degree. C., the loop frequency being very sensitive to temperature and to the necessity for strict control of the quality of production of pure progressive Rayleigh waves, any parasite reflection in this looping method disturbing the measurements. The solution of these problems necessitated a considerable amount of electronic equipment, high performances and the permanent presence of a highly skilled operator.
Finally, interpretation of the results was difficult since the two quantities measured, namely variations of frequency and attenuation, were connected.